It is possible to measure the surface temperature of the Sun by direct experiment.

The Sun is a big ball of hot plasma (ionized gas) that holds itself up by fusing hydrogen nuclei into helium nuclei in its central regions. In order for these reactions to take place, the temperature in the core has to be about 13 million kelvin. We can use our theoretical understanding of the way the Sun works to model the actual central temperature: the current number is 15.71 million kelvin.

The sunlight that we see is "made" deep in the Sun's interior - it then bounces around inside the Sun and gradually makes its way out. We call the surface of the Sun as the mean distance from the core at which the light bounces for the last time before coming out (remember that unlike the Earth, the Sun has no solid surface, so we have to define it somehow!). It turns out that the light we see from the surface has nearly the spectrum of a blackbody (A blackbody is a physical term that means that the fraction of light that is emitted at each wavelength is a function of the surface temperature alone. Ref.: http://astronomy.swin.edu.au/cosmos/b/blackbody+radiation) This is great for us, because it means that we can measure the spectrum of the Sun, and then fit it with a blackbody spectrum to derive the temperature. For a picture of the Sun's spectrum, look here.

From the solar spectrum, we can infer that the surface temperature of the Sun is about 5800 kelvin, or 5527 degrees celsius.

About the Author

Kristine studies the dynamics of galaxies and what they can teach us about dark matter in the universe. She got her Ph.D from Cornell in August 2005, was a Jansky post-doctoral fellow at Rutgers University from 2005-2008, and is now a faculty member at the Royal Military College of Canada and at Queen's University.

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